1. Field of the Invention
This invention generally relates to a device for developing an electrostatic latent image, and in particular, to a developing device using a single component developer. More specifically, the present invention relates to a developing device which is particularly suitable for use with a non-magnetic, single component developer.
2. Description of the Prior Art
In the dry-type development method, which is applied in various imaging apparatuses, such as electrophotographic copiers and electrostatic recorders, there are two types of developer to be used: a two component developer containing toner particles and carrier beads and a single component developer containing toner particles but no carrier beads. The usage of a two component developer allows to obtain a developed image excellent in quality relatively stably; however, there are such disadvantages as difficulty in maintenance and compactization since the carrier beads are repetitively used so that they tend to deteriorate in performance, and, moreover, the mixture ratio between the toner particles and the carrier beads must be maintained at constant at all times, otherwise, the density of the resulting developed image would fluctuate.
In the case of the single component developer, there are no such disadvantages which are normally encountered when use is made of the two component developer as described above. However, when the single component developer is to be used, it is required that the single component developer be stably formed into a thin film uniform in thickness in order to have the toner particles charged sufficiently as well as uniformly. For this reason, it has been proposed to use a doctor blade as pressed against a developing sleeve. With this structure, since the developing sleeve is driven to rotate in a predetermined direction, the toner particles are partly pinched between the outer peripheral surface of the developing sleeve and the doctor blade so that the toner particles are thereby charged and formed into a thin film. In this case, however, since the doctor blade is kept pressed against the developing sleeve at a relatively strong force, that portion of the doctor blade which is in scrubbing contact with the outer peripheral surface of the sleeve gradually wears out so that the performance of the doctor blade necessarily deteriorates. If this happens, the resulting film of toner particles becomes irregular in thickness and thus in the amount of charge, which then causes a deterioration in the developing performance.
Moreover, the toner particles of the prior art single component developer were magnetically attracted to the outer peripheral surface of the developing sleeve. Thus, the toner particles of the prior art single component developer were required to contain magnetic powder and the developing device using such a prior art single component developer were required to include magnets as disposed inside of the developing sleeve so as to have the magnetic toner particles magnetically attracted to the outer peripheral surface of the developing sleeve. Such requirements necessarily would make the developing device larger in size and complicated in structure. In addition, the selection of materials for the toner particles were necessarily limited.